chest fly is a great way for isolation chest more and doing less work with shoulder and triceps, but at the same time, this exercise requires good biceps strength and endurance.
just because there is a machine with manuals for targeting a muscle, you should not follow the instructions exactly as they are and need to adjust it based on your goals, and more importantly, your anatomic flexibility and ability. every small detail matters.
once you know how to set yourself and machine up, you are good to go as long as the machine is the same and there is no change in your flexibility(like an injury which can restrict your motion).
in this video, like always, i go from your toes all the way up to your head and tell you how you should position them and what points you should keep in mind in order to perform this workout the best way, and also how to keep yourself safe.
remember everyone is different and there is still some more details to this exercise so if you felt like you are doing everything mentioned in this video and still feel weird, either contact me or find a certified and reliable PT to help you out. everyone is different and there are always some exceptions in any topic.
make sure throughout the exercise you feel the tension on chest muscle and not the joint and tendons. if the pain is persistent, especially during rest periods between sets, something is wrong, but if the tension is just on the muscle and it is just hard because of effort levels and not too much stretch, you are good to go.
anyway, make sure the weight is appropriate so you have full control over the weight and avoid going fast. it is recommended to do this exercise between 6 to 15 reps for best results but make sure the form is good during all the reps. it is ok to go a little bit lighter and do more reps but avoid doing less than 6 reps with heavier weight.